Laser-beam levelling instruments of the above-mentioned type serve to determine a horizontal plane or a plane inclined by a desired percentage in at least one direction. In the course of this, the laser beam, which generally emerges invisibly from the instrument, is caused to rotate horizontally by a rotary head. The rotating laser beam can then be received by a receiver and made visible at any desired point around the levelling instrument. Such a laser-beam levelling instrument enables a large number of different surveying and checking tasks to be carried out. The type of instrument here in question, wherein the laser plane can be inclined in two axes, is used primarily in construction work below ground level and, for example, during the levelling of large agricultural areas. Since the reference plane radiated by the laser can be inclined as desired in two axes, that is to say in a transverse gradient and in a longitudinal gradient, an area of ground, whether it be for a car park, a football field or an agricultural field, can be brought to a predetermined transverse and longitudinal gradient, for example in order to ensure the drainage. In this case, checking work can either be done by means of the laser-beam receiver secured to a surveyor's rod or better still the grading work can be controlled by means of a receiver mounted directly on the grading machine.
In former inclinable laser-beam levelling instruments the inclination of the laser reference plane has generally been effected by means of an intermediate member between a tripod and the instrument. In this case, the whole instrument is inclined and the inclination can only be effected in one direction. Now in order to be able to measure the transverse gradient also, the instrument is turned through 90.degree. on the intermediate member. During this rotation, the instrument executes a pendulum-like movement, the lowest point of the pendulum movement being in the middle, that is to say at 45.degree.. If the transverse inclination is to be effected by a different value from the longitudinal inclination, however, a third inclination corresponding to the lowest point of the said pendulum movement must be adjusted at the instrument. According to the two values desired of the transverse and longitudinal gradients respectively, this third value will no longer be in the middle of the 90.degree. angle but nearer to either one end or the other. In order to achieve the particular transverse and longitudinal gradients required, it is necessary to calculate, with reference to a table, in what angular position the third auxiliary inclination value has to be set. This is naturally somewhat complicated.